This is an uncommonly beautiful image of M101, the Pinwheel Galaxy, captured by photographer John Purvis on a clear night. It's a rare gem, because M101 is especially difficult to photograph, being so far away and so faint. It requires long exposures, and a lot of patience.
As Purvis explains on his Flickr page:
The beautiful face-on spiral galaxy M101 in Ursa major is fantastically detailed but dim! It's only 8th magnitude so it requires long exposures and relatively dark skies.
The images below are composed of 7 x 12 minute (720 sec) and 2 x 6 minute (360 sec) exposures (total integration time = 94 minutes).
M101 has slightly eccentric spiral arms and has many bright nebulae (bright blue-white OB class star clusters and surrounding gas) and reddish HII regions. These features suggest that it has interacted with another galaxy gravitationally and is currently undergoing very active star formation. There are also several background galaxies in the full frame image as a large cluster of galaxies lies in the direction of Ursa major.
Although Charles Messier's assistant, Pierre Mechain was the first to catalogue M101, It was Lord Rosse in Ireland (with the 72 inch Leviathan telescope at Birr castle) who was the first to analyse and catalogue some of the subregions listed below.
M101 was actually listed twice in the Messier catalogue as M101 and M102 in 1781 (Pierre's mistake! Corrected in 1783) hence you will sometimes hear the Messier catalogue listed as either 110 or 109 objects.
The individual HII regions and emission nebulae in M101 have their own NGC numbers. M101 has eleven NGC entries – more than any other single entity.
See more of Purvis' photos here.